With the progress of high-performance internal combustion engines, a center electrode or ground electrode of a spark plug for such an internal combustion engine tends to be used at higher temperatures. Since the material of such an electrode may be degraded through heat accumulation by combustion, the electrode is required to have high thermal conductivity for achieving good heat dissipation. Therefore, there has been proposed employment of an electrode including an outer shell formed of a nickel alloy exhibiting excellent corrosion resistance, and a core formed of a metal having a thermal conductivity higher than that of the nickel alloy <see, for example, Japanese Patent Application Laid-Open (kokai) No. H05-343157>.